In SIP based Voice over Internet Protocol (VoIP) networks, SIP-ISUP interworking units (often called Media Gateway Controller Functions, MGCF) are commonly used. The MGCF handles call processing functions and message translations between SIP messages in an Internet Protocol (IP) network and ISUP messages in a Public Switched Telephone Network (PSTN) network. Most MGCF products are implemented in compliance with standards such as IETF RFC 3398 or ANSI T1.679.
In accordance with SIP standards in an IP network, the MGCF follows specific call processing rules and call flows for SIP to ISUP calls. The call flows generally require the MGCF to wait for an Address Complete Message (ACM) response from the PSTN network. Upon receiving the ACM, the MGCF can process a status indicator in the ACM and respond to a SIP client with call status and progress. After responding to the SIP client, the MGCF can proceed to request, or cut, a media path to the PSTN network. The media path allows for delivery of media content to the SIP client from the PSTN network.
In accordance with ISUP standards in a PSTN network, however, the originating switch is required to cut a receive only (RECVONLY) media path immediately upon sending out an Initial Address Message (IAM) message. The media path is established immediately afterwards because some ISUP trunks send audio information back to the caller immediately after receiving the IAM message.
This aspect of cutting a RECVONLY media path in the PSTN network immediately after the sending out an IAM message conflicts with the call flow logic of SIP and produces an interworking incompatibility between SIP and ISUP. In particular, the waiting period associated with the processing of the ACM message per SIP standard in the IP network prevents early media from being received by the SIP client since a media path has not yet been cut to the PSTN network.
Accordingly, any messages transmitted by the PSTN network to the MGCF prior to the establishment of the media path may be dropped at the MGCF. These messages may not reach the SIP client. Under these circumstances, the caller may not be able to receive information from the network regarding the call attempt.